The atmospheric diffuser is a normally very reliable and effective apparatus used for the treatment of paper pulp, typically for washing or bleaching the pulp. Atmospheric diffusers have been sold by Kamyr, Inc. of Glens Falls, New York for about 20 years and have performed effectively in a wide variety of circumstances. In a typical atmospheric diffuser, annular screens are mounted by extraction arms concentric with the vertical axis of the diffuser vessel, and are reciprocated up and down within the vessel, matching the flow speed of the pulp in the upstroke, and moving quickly in the downstroke to effect backflushing of the screens to preclude them from clogging. Nozzles for introducing the wash or bleach liquid are mounted on a scraper arm at the top of the vessel, and if a multi-stage diffuser is provided at another treatment arm at the bottom of each stage, and extend between the annular screens, introducing treatment liquid which flows through the pulp and then displaces liquid within the pulp, which is withdrawn through the extraction arms.
One of the few significant problems that can occur in operation of Kamyr atmospheric diffusers is nozzle breakage. As the treatment arms are rotated about the central vertical axis of the vessel, the nozzles are subjected to a drag force by the pulp. For the outer nozzles on the arm, which have a faster tangential velocity than the interior nozzles, this drag force can become quite significant, especially if some of the nozzles become clogged or another aberrant condition occurs that causes the resistance of the pulp at that particular nozzle to be greater than expected. If a nozzle breaks as a result of this drag force, pieces of the nozzle may affect the operation of the diffuser. In a number of cases physical damage to the interior of the diffusers was wrought by floating pieces of broken nozzle. Operation of the diffuser must be arrested and the nozzle replaced, and other adverse consequences can occur. The down time for replacing the nozzles can be significant.
According to the present invention, a method and apparatus are provided which substantially preclude nozzle breakage. According to the present invention, a connection between the nozzle and the treatment arm will become interrupted, rather than the nozzle breaking. This means that nozzle pieces will not break off into the diffuser creating a potential hazard, and the nozzles themselves need not be replaced in order to repair the diffuser.
In the preferred embodiment of the invention, the connection between the nozzle and the treatment arm assembly is provided by a nozzle extension having a pivot pin connection at one side end thereof, and a shear pin connection at the other side end thereof, to the arm assembly. A first end of the nozzle is integral with the extension, with the second end of the nozzle--containing the fluid introduction openings therein--vertically remote from the extension. The shear pin preferably has shear resistance properties that are about 50% of those of the nozzle, so that if the drag force on the nozzle is about 50% of the level that will break the nozzle, the shear pin will sever instead. It is a simple and inexpensive procedure to replace the shear pin compared to replacing the entire nozzle.
In order to prevent the second end of the nozzle from contacting one of the annular screens of the diffuser, the amount of pivotal movement of the nozzle once the shear pin fractures is limited, typically to about 15.degree.-26.degree.. This action is normally self-correcting for the condition that caused the drag force that resulted in shear pin fracture. Once the nozzle pivots away from the treatment arm, treatment fluid will flow freely into the pulp at that point, diluting the pulp substantially and thereby decreasing the drag force. This free flow of treatment fluid will also cause a pressure drop in the treatment fluid supply system, which can be noted by an operator who can then schedule the appropriate repair procedures.
It is the primary object of the present invention to minimize the possibility of nozzle breakage in an atmospheric diffuser. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.